Cholesterol is one of the building blocks of the cell and is absolutely required for normal cellular function, but its excess often leads to abnormal proliferation, migration, inflammatory responses and/or cell toxicity. Removal of cholesterol from the cells to apoA-I and to HDL, an apoA-I-containing lipoprotein, is mediated by ATP-binding cassette transporters (ABC), predominantly ABCA1 and ATP-binding cassette sub-family G member 1 (ABCG1) (1-3). Recent reports that ABCA1/G1-deficient hematopoietic cells tend to hyperproliferate (4-6) and that ABCG1 and HDL protect against endothelial dysfunction in hypercholesterolemic mice (7) suggest the importance of efficient cholesterol efflux in maintaining cellular function.
ApoA-I binding protein (AIBP) is a secreted protein discovered in a screen of proteins that physically associate with apoA-I (8). The human AIBP gene (APOA1BP) is located at 1q22. 1q21.2 on chromosome 1, which corresponds to the 1q21-q23 locus for familial combined hyperlipidemia, a common, multifactorial and heterogeneous dyslipidemia predisposing to premature coronary artery disease (9). This chromosomal location of the gene and the protein binding to apoA-I, the primary apolipoprotein involved in removal of cholesterol from cells, prompted the hypothesis that AIBP is involved in cholesterol efflux (8, 10). However, a function of AIBP in cholesterol metabolism has not been experimentally tested.